Low-temperature magnetotransport measurements were performed on AlxGa1-xN/GaN two-dimensional electron systems. By studying the beating pattern in the Shubnikov–de Haas oscillations in a perpendicular magnetic field, we are able to measure the zero-field spin-splitting energies in our systems. Our experimental results demonstrate that the Rashba term due to structural inversion asymmetry is the dominant mechanism which gives rise to the measured zero-field spin splitting in our wurzite AlGaN/GaN structures. By utilizing the persistent photoconductivity (PPC) effect, we are able to increase the carrier density n in our AlGaN/GaN two-dimensional electron system. It is found that the Rashba spin-orbit splitting parameter α decreases with increasing n. We suggest that the formation of long-lived electron-hole pairs induced by the PPC effect decreases the large electric field near the AlGaN/GaN interface, causing α to decrease with increasing n.